Talk about K2V specifics

doc/talks/2023-01-18-tocatta/talk.tex

@@ -780,17 +780,73 @@
 \begin{frame}
 	\frametitle{K2V Design}
 	\begin{itemize}
-		\item A new, custom, minimal API
+		\item A new, custom, minimal API\\
+			\vspace{.5em}
+			\begin{itemize}
+				\item Single-item operations
+				\item Operations on ranges and batches of items
+				\item Polling operations to help implement a PubSub pattern
+			\end{itemize}
 			\vspace{1em}
 		\item<2-> Exposes the partitoning mechanism of Garage\\
 			K2V = partition key / sort key / value (like Dynamo)
 			\vspace{1em}
-		\item<3-> Coordination-free, CRDT-friendly (inspired by Riak)\\
+		\item<3-> Weakly consistent, CRDT-friendly\\
+			$\to$ no support for transactions (not ACID)
 			\vspace{1em}
 		\item<4-> Cryptography-friendly: values are binary blobs
 	\end{itemize}
 \end{frame}
 
+\begin{frame}
+	\frametitle{Handling concurrent values}
+	\textbf{How to handle concurrency?} Example:
+	\vspace{1em}
+	\begin{enumerate}
+		\item Client $A$ reads the initial value of a key, $x_0$
+			\vspace{1em}
+		\item<2-> Client $B$ also reads the initial value $x_0$ of that key
+			\vspace{1em}
+		\item<3-> Client $A$ modifies $x_0$, and writes a new value $x_1$
+			\vspace{1em}
+		\item<4-> Client $B$ also modifies $x_0$, and writes a new value $x'_1$,\\
+			without having a chance to first read $x_1$\\
+			\vspace{1em}
+			$\to$ what should the final state be?
+	\end{enumerate}
+\end{frame}
+
+\begin{frame}
+	\frametitle{Handling concurrent values}
+	\begin{itemize}
+		\item If we keep only $x_1$ or $x'_1$, we risk \textbf{loosing application data}
+			\vspace{1.5em}
+		\item Values are opaque binary blobs, \textbf{K2V cannot resolve conflicts} by itself\\
+			(e.g. by implementing a CRDT)
+			\vspace{1.5em}
+		\item Solution: \textbf{we keep both!}\\
+			$\to$ the value of the key is now $\{x_1, x'_1\}$\\
+			$\to$ the client application can decide how to resolve conflicts on the next read
+	\end{itemize}
+\end{frame}
+
+\begin{frame}
+	\frametitle{Keeping track of causality}
+	How does K2V know that $x_1$ and $x'_1$ are concurrent?
+	\vspace{1em}
+	\begin{itemize}
+		\item $read()$ returns \textbf{a set of values} and an associated \textbf{causality token}\\
+			\vspace{1.5em}
+		\item When calling $write()$, the client sends \textbf{the causality token from its last read}
+			\vspace{1.5em}
+		\item The causality token represents the set of values \textbf{already seen by the client}\\
+			$\to$ those values are the \textbf{causal past} of the write operation\\
+			$\to$ K2V can keep concurrent values and overwrite all ones in the causal past
+			\vspace{1.5em}
+		\item Internally, the causality token is \textbf{a vector clock}
+	\end{itemize}
+\end{frame}
+
 \begin{frame}
 	\frametitle{Application: an e-mail storage server}
 	\begin{center}
@@ -800,7 +856,7 @@
 
 \begin{frame}
 	\frametitle{A new model for building resilient software}
-	\begin{itemize}
+	\begin{enumerate}
 		\item Design a data model suited to K2V\\
 			{\footnotesize (see Cassandra docs on porting SQL data models to Cassandra)}
 			\vspace{1em}
@@ -810,22 +866,16 @@
 				\item Store opaque binary blobs to provide End-to-End Encryption\\
 			\end{itemize}
 			\vspace{1em}
-		\item Store big blobs (files) in S3
+		\item Store big blobs (files) using the S3 API
 			\vspace{1em}
 		\item Let Garage manage sharding, replication, failover, etc.
-	\end{itemize}
+	\end{enumerate}
 \end{frame}
 
 \begin{frame}
 	\frametitle{Research perspectives}
 	\begin{itemize}
-		\item Write about Garage's global architecture \emph{(paper in progress)}
-			\vspace{1em}
-		\item Measure and improve Garage's performances
-			\vspace{1em}
-		\item Discuss the optimal layout algorithm, provide proofs
-			\vspace{1em}
-		\item Write about our proposed architecture for (E2EE) apps over K2V+S3
+		\item TODO
 	\end{itemize}
 \end{frame}